Microorganisms contain a variety of cryptic genes, genes that are not normally exposed but which can be activated by mutational events and genetic rearrangements. Although silent, these genes are not destroyed by mutational inactivation. We have recently discovered a system of cryptic genes for cellobiose utilization in E. coli. We propose to use this system as a model to study the evolution, molecular biology, and maintenance of cryptic genes. We will investigate this problem at the molecular and biochemical levels in the laboratory strain, E. coli K12. In that study we shall investigate the structure, function and mechanism of activation of the cryptic cellobiose genes. We shall apply that information to population biology studies of natural E. coli isolates in order to understand a) the basis of selection that maintains silent genes in populations in the face of mutational pressure, and b) the generality of molecular mechanisms that silence and activate cryptic genes.